On Wed, Jan 19, 2011 at 10:00 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: > Nick Piggin <npiggin@xxxxxxxxx> writes: > >> On Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <jack@xxxxxxx> wrote: >>> Hi, >>> >>> On Tue 18-01-11 10:24:24, Nick Piggin wrote: >>>> On Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: >>>> > Nick Piggin <npiggin@xxxxxxxxx> writes: >>>> >> Do you agree with the theoretical problem? I didn't try to >>>> >> write a racer to break it yet. Inserting a delay before the >>>> >> get_ioctx might do the trick. >>>> > >>>> > I'm not convinced, no. The last reference to the kioctx is always the >>>> > process, released in the exit_aio path, or via sys_io_destroy. In both >>>> > cases, we cancel all aios, then wait for them all to complete before >>>> > dropping the final reference to the context. >>>> >>>> That wouldn't appear to prevent a concurrent thread from doing an >>>> io operation that requires ioctx lookup, and taking the last reference >>>> after the io_cancel thread drops the ref. >>>> >>>> > So, while I agree that what you wrote is better, I remain unconvinced of >>>> > it solving a real-world problem. Feel free to push it in as a cleanup, >>>> > though. >>>> >>>> Well I think it has to be technically correct first. If there is indeed a >>>> guaranteed ref somehow, it just needs a comment. >>> Hmm, the code in io_destroy() indeed looks fishy. We delete the ioctx >>> from the hash table and set ioctx->dead which is supposed to stop >>> lookup_ioctx() from finding it (see the !ctx->dead check in >>> lookup_ioctx()). There's even a comment in io_destroy() saying: >>> /* >>> * Wake up any waiters. The setting of ctx->dead must be seen >>> * by other CPUs at this point. Right now, we rely on the >>> * locking done by the above calls to ensure this consistency. >>> */ >>> But since lookup_ioctx() is called without any lock or barrier nothing >>> really seems to prevent the list traversal and ioctx->dead test to happen >>> before io_destroy() and get_ioctx() after io_destroy(). >>> >>> But wouldn't the right fix be to call synchronize_rcu() in io_destroy()? >>> Because with your fix we could still return 'dead' ioctx and I don't think >>> we are supposed to do that... >> >> With my fix we won't oops, I was a bit concerned about ->dead, >> yes but I don't know what semantics it is attempted to have there. >> >> synchronize_rcu() in io_destroy() does not prevent it from returning >> as soon as lookup_ioctx drops the rcu_read_lock(). >> >> The dead=1 in io_destroy indeed doesn't guarantee a whole lot. >> Anyone know? > > See the comment above io_destroy for starters. Note that rcu was > bolted on later, and I believe that ->dead has nothing to do with the > rcu-ification. Ah, yep, that makes sense indeed. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
